The 10kg (22 lb) limit for carry on seems to be pretty universal. But allowed size of carry on luggage seems to vary. Ryan air states 55cm x 40cm x 20 cm. (21.5" x 15.7" x 7.8") That's smaller than any US airline.
Various sites and blogs talk about un-even policy administration about carry on. I have two Ryan flights planned. Should I worry about having to shell out an additional $50 bucks per flight just because my darn carry on bag has wheels that stick out? My new Ricardo bags (cheap at Costco) are tall by the diameter of the glide wheels.